1. Field of the Invention
The present invention relates to cementing compositions and methods for cementing oil and gas wells and, more specifically, to method and composition for cementing pipes for oil and gas wells where the pipes have opposing ends and the temperature differential between the two ends of the pipe within the well borehole is as much as 75.degree. F., or more.
2. Description of the Prior Art
Hydraulic cements are cements that can set under water. This setting property serves a variety of purposes. Hydraulic cements are often used in placement of pipes or casings within a well bore of a subterranean formation for the construction of oil and gas wells. In the oil and gas industry, successful cementing of well pipe and casing during oil and gas well completion requires cement slurries having several important properties. The slurry must have a pumpable viscosity, fluid loss control, minimized settling of particles and the ability to set within a practical time. Polymers, such as hydroxyethyl cellulose, carboxymethylhydroxyethyl cellulose, polyvinyl alcohol and polysulfonated polymers are commonly used to provide these important properties.
In a typical completion operation, the cement slurry is pumped down the inside of the pipe or casing and back up the outside of the pipe or casing through the annular space. This seals the subterranean zones in the formation and supports the casing. The amount of water used in forming the cement slurry depends upon the type of hydraulic cement selected and the job conditions at hand. The amount of water used can vary over a wide range, depending upon such factors as the required consistency of the slurry and upon the strength requirement for a particular job.
These completion procedures often place the hydraulic cement within or next to a porous medium, for example, earthern strata in the well bore. When this happens, water tends to filter out of the slurry and into the strata during placement and setting of the cement. Many difficulties relate to an uncontrolled fluid loss of this type, such as an uncontrolled setting rate, premature gelation of the slurry, bridging of the annular space between the formation and the casing, improper placement of the slurry, impaired strength properties and a contamination of the surrounding strata. These conditions are all undesirable in oil and gas well cementing operations. Special additives have consequently been designed to control fluid loss during well cementing operations.
To achieve a satisfactory primary cementing job, it is also important to achieve a tight bond between the pipe or casing and the cement sheath to prevent vertical communication of fluids or gas along or within the cement column. In order to achieve a tight bond, it is important to reduce the cement matrix permeability and retain water during the initial set, effectively blocking the porous cement structure.
In cementing certain long strings of pipe or casing, the temperature differential between the two ends of the pipe might be 75.degree. F., or more. In such cases, a slurry design must be adequate to cement a pipe in a gas or oil well where the two ends of the pipe are, for example, at 310.degree. F. and 235.degree. F., respectively. One problem presented by such a wide temperature differential is that the slurry is required to remain in a pumpable state, for example, for two hours (job time plus safety time) at the lower, relatively high temperature zone (310.degree. F. bottom hole circulating temperature, BHCT, in this example), and yet develop sufficient compressive strength at the upper, relatively low temperature zone (235.degree. F. in this example) to minimize the rig time. For purposes of this example, the requirements are pumping time of about 2 hours 30 minutes .+-.15 minutes at 310.degree. F. BHCT and 16100 psi pressure at a 16150 foot depth, while developing a compressive strength of at least 200 psi in twelve hours. The slurry design must also be adequate to cement two such well zones at very wide temperature differentials through the use of a single slurry, rather than through the use of staged cement slurries.
Traditional API Class "H" cement systems would generally not be able to achieve these desired results for several reasons. Typical long strings of pipe of the type under consideration use slurries whose strength development can be somewhat slow, or even non-existent, when retarded for bottom hole conditions and cured at lower temperatures. Additionally, given the long exposure to annular conditions experienced by the lead portion of the cement column being pumped, fluid loss control is a necessity. Since many of the fluid loss additives tend to retard cement setting times and strength development (especially when loaded for bottom hole circulating temperature and then cured under surface conditions), the addition of fluid loss additives only serves to exaggerate the nature of the problem.
Thus, a need exists for an improved well cementing composition which provides a slurry having a pumpable viscosity, adequate fluid loss control, minimized settling of particles and the ability to set within a practical time and develop sufficient compressive strength, even where well zones at wide temperature differentials are being cemented.
A need also exits for such an improved cement composition which allows the transition from a hydrostatic pressure transmitting liquid to a set cement, with very little, if any, time spent in the plastic, self-supporting/gas-migrating stage.